1. Field of the Invention
Embodiments disclosed herein generally relate to a method for seasoning a sputtering target in-situ with a substrate to be processed.
2. Description of the Related Art
The electron mobility of a semiconductor layer has a very strong effect on the speed of the device and the current which may be driven through the device. The higher the electron mobility, the faster the speed of the device and the higher the source-drain current under the same voltage. In recent years, amorphous silicon and polysilicon have been the semiconductor materials of choice for field effect transistors (FETs) such as thin film transistors (TFTs), for backplane to drive liquid crystal displays (LCDs), organic light emitting diode (OLED) displays, quantum dot displays, and building solar cell panels. Amorphous silicon may have an electron mobility as high as about 1 cm2/V-s. Low temperature polysilicon may have an electron mobility higher than 50 cm2/V-s, but requires a complicated process step such as laser annealing to achieve the electron mobility. Therefore, the cost of producing polysilicon with an electron mobility higher than 50 cm2/V-s is very high and not suitable for large area substrate applications.
In an FET, the semiconductor material creates the channel between the source and drain electrodes. Without a voltage supply to the gate electrode, no current may go through the source-drain electrodes even with a voltage between the source-drain electrodes. As voltage is supplied to the gate electrode, mobile electrons inside the semiconductor layer will accumulate in the area very close to the interface between the gate dielectric layer and the semiconductor layer. The semiconductor layer becomes conductive, and electrons may go through the source-drain electrode easily with a low voltage between the source-drain electrodes. High mobility of the semiconductor materials indicates the mobile electrons in the semiconductor are more sensitive to the electric field created by the gate electrode, and the semiconductor channel becomes more conductive. The semiconductor material determines the current which may go through the semiconductor channel influenced by voltage applied across the gate and source terminals. The greater the mobility of the semiconductor material, the less voltage is needed to achieve the current required across the FET.
Amorphous silicon may rely upon hydrogen passivation to achieve a desired mobility in a TFT. The amorphous silicon may be deposited by chemical vapor deposition (CVD) at temperatures up to about 350 degrees Celsius. The hydrogen passivation, while helping the amorphous silicon achieve the desired mobility, may not be stable such that a TFT's threshold voltage may change with time under gate electrode voltage and under relatively high temperatures created by the device itself.
In recent years, transparent TFTs have been created in which zinc oxide has been used as the active channel layer. Zinc oxide is a compound semiconductor that can be grown as a crystalline material at relatively low deposition temperatures on various substrates such as glass and plastic.
Sputtering may be used to deposit the active channel for TFTs. During a sputtering operation, material may deposit not only on the substrate, but also along the other exposed surfaces of the processing chamber. The deposition on the surfaces could change the process chamber since the deposition may change the process chemistry by consuming or releasing reactants, or the deposition may change the plasma condition due to conductivity changes. Therefore, the processing chamber is periodically off-line to permit the chamber to be cleaned. When the processing chamber is off-line, substrate throughput is diminished.
Therefore, there is a need in the art to produce a stable, high quality semiconductor material having sufficiently high mobility not only on glass substrates with high process temperatures, but also on plastic substrates and other flexible substrates. There is also a need in the art for a sputtering process that reduces the negative impact of chamber cleaning.